Apparatus and method for detonating secondary explosives



Feb. 18, 1964 J. WENOGRAD 3,121,390

APPARATUS AND METHOD FOR DETONATING SECONDARY EXPLOSIVES Filed Jan. 2, 1962 VOLTAGE PULSER ii I I B R2 w, TRIGGER FIG.2.

EXPLOSION E=250mvo 5 000 TIME (0 see) X 'FIGA. A

' INVENTOR. 200 400 e00 500 I000 JOSEPH WENOGRAD TEMP. C BY ATTYS.

United States Patent The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payinent of any royalties thereon or therefor.

This invention relates generally to the field of the assessment of the relative impact sensitivity of liquid explosives and propellants and of meltable solid explosives and propellants and more particularly relates to an apparatus for determining the delaytime to explosion of these explosives and propellants as a function of temperature applied thereto at very high temperatures and toa new initiator and method for initiating high explosives.

In the past, it was believed that a wide variety of physical, chemical and mechanical properties determined the absolute value of an explosives impact sensitivity. More recently, however, it has been proposed that only one of these properties, i.e. the thermal decomposition velocity at very high temperatures believed to prevail in hot spot initiation centers, varies sufiiciently among the organic high explosives to account for the differences in the relative impact sensitivity which have been observed. In the temperature and time ranges encountered in impact sensitivity studies, the chemical decomposition of organic explosives is so rapid compared to the heat transfer process so as to virtually preclude any isothermal measurements on the condensed explosives. Further, these temperatures are sufficient to cause a very rapid vaporization of most organic materials resulting in the formation of an insulating layer between the material and the source of thermal energy. Therefore, in order to ascertain the behavior of explosives in the temperature range of interest, that is, the temperature range from 3001,000 C., heat must be applied to the explosive very rapidly while the explosive is held under heavy confinement to prevent vaporization and to contain the "explosive in the hot region.

In the past, in order to assess the relationship between explosion time and the temperature involved, the explosive was enclosed in a metal or glass tube and immersed in a hot bath of molten metal or oil. Such a method involves relatively long time delays while heat is being conducted through the walls of the container and for this reason, the temperature history of the explosive sample as a whole is relatively uncertain. In these experiments, the materials tested have not been heavily confined and therefore the possibility of vaporization of the explosive from the hot region before explosion is quite likely. Time delays involved in these old methods were rather long, generally being greater than 0.1 second and there has been in the past no reliable means of measurement of time delay at extreme temperatures where the explosion takes place in less than 0.1 second.

A second old method of measuring explosive sensitivity is the so called drop test method which is accomplished by dropping a known weight from a known height upon an explosive sample and observing whether or not the explosive is exploded. This method has the disadvantage of depending upon an extremely non-reproducible and poorly understood degradation of mechanical into thermal energy. Further, the individual experiment yielded only a fire or a no-fire result and a large number of shots had to be statistically analyzed to yield dependable results.

In the past, it has further been the practice to initiate a secondary explosive by first initiating a very sensitive and hazardous primary explosive arranged in close proximity to the secondary explosive. In initiation by electrical methods such, for example, as the hot wire type initiator, the sensitive primary explosive necessitates elaborate safety devices, such, for example, as the various means utilized to short the hot wire until shortly before detonation is desired to prevent detonation of the very sensitive primary explosive by heating of the wire due to stray electric and magnetic fields. The use of a primary explosive has been necessitated by the lack of a convenient and dependable method of direct initiation of a secondary explosive by electrical means.

The apparatus described herein permits the construction of an explosive device using only secondary explosives and, because of the high energies required to detonate the relatively insensitive secondary explosive, the elaborate safety devices necessary when sensitive primary explosives are used are not required.

It is therefore, a primary object of this invention to provide an apparatus for directly initiating a secondary explosive.

Another object of the invention is to provide a new and improved apparatus for initiating explosives by the application of very high temperatures.

A further object is the provision of an apparatus for measuring the time delay to explosion of an explosive subjected to a high temperature.

Still another object is to provide an apparatus capable of measuring the time delay to explosion of an explosive when subjected to a high temperature when the reaction time is in the microsecond region.

A still further object is the provision of an initiator utilizing only secondary explosives.

A further object is to provide a new method of initiating secondary explosives without the use of a primary explosive.

Still another object is to provide an initiator which does not necessitate the use of safety means to prevent initiation by extraneous radiant energy.

Other objects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIG. 1 is a schematic diagram of a complete electrical system according to a preferred embodiment of the invention;

FIG. 2 is a typical trace obtained on the oscilloscope of FIG. 1 when the circuit thereof is initiated;

FIG. 3 is a typical.resistance-temperature curve utilized to determine the temperature of the explosive at the time of explosion; and

FIG. 4 is a diagrammatic view of the invention as an initiator for use in the initiation of secondary explosives.

To accomplish the foregoing objects the invention broadly contemplates the loading of an explosive into a fine metal tubing such, for example, as that utilized in hypodermic needles, the tubing being utilized as one leg of a Wheatstone bridge and is then heated, substantially instantaneously, by the discharge of a capacitor. The voltage across the output terminals of the bridge during this period is placed across an oscilloscope and the trace recorded by an osoillographic recording camera. Upon explosion of the explosive, an abrupt change occurs in the voltage trace from which both the resistance of the tube under test and the time delay to explosion can be computed.

The major difficulty hindering the observation of explosives subjected to very high temperatures, that is, the volatilization of the explosive at these temperatures, has

een overcome by enclosing them in short lengths of fine metallic tubing. The tubing may preferably be made of stainless steel but may be made of any electrically conducting material having a resistance high enough to produce the desired temperature. The tubing preferably has a nominal outside diameter of approximately 0.014 inch and an inside diameter of 0.007 inch, the low heat capacity and rather high resistivity of this thin wm led tubing permits it to be heated electrically to very high temperatures rapidly. The explosive, contained within the tubing, being a non-conductor passes no electrical current but is heated by conduction. The small, but significant, temperature coeificient of resistance of the tubing permits its temperature to be determined through a resistance measurement as will hereinafter be more fully explained. Although the tubes are fine, their construction and geometry are such that they have sufiicient strength to provide the heavy confinement required to prevent volatilization of the explosive at the high temperatures involved. Further, although fine, the tube has sufficient mass that its temperature change due to the heat loss to the surrounding air is quite small in the time required to explode the explosive.

The individual experimental tubes may be made by soft soldering the tubing to binding posts which can be mounted into the circuit of FIG. 1 with a pair of set screws. Before loading the tubes with expiosives, they are annealed to a constant resistance by heating them to a cherry red heat by passing about two amperes of current through them. The resistance R of the tubes at room temperature is determined by measuring the voltage drop across them on the passage of a very low current. The tubes used were 2 to 2 /2 inches in length between the binding posts and had a resistance of approximately 0.6 ohm. The resistivity-temperature curve for these tubes, illustrated in FIG. 3, may be determined by measuring the resistance of the tubes at known temperatures as will hereinafter be more fully described.

In order to fill the tubes with explosives, they are prepared with one end open and one end closed, the open end of the tubing being rested against the bottom of a small container containing a small amount of the explosive to be tested. Liquid explosives are placed in a vacuum desiccator and solid explosives in a vacuum oven regulated to a temperature a few degrees above their melting point. The entire system including the tube is then evacuated and the explosive is forced into the tube by the readmission of air to the system. The open end is then closed upon the filled explosive.

Although some explosives are markedly unstable at or slightly above their melting points, the extent of decomposition in the short time they must remain at this temperature in order to fill the tube is believed to be insufiicient to effect the results obtained.

Referring now to FIG. 1 there is illustrated an apparatus for subjecting the sample of explosive to be tested to a high temperature and observing the time delay to explosion thereof at this temperature. The apparatus comprises a high voltage pulser 11 having a Wheatstone bridge 12 and the series combination of resistance R and storage battery E conencted in parallel across its output. Bridge circuit 12 comprises a pair of non-inductive resistances R and R a constant resistor R and a tube as resistance R Tube R is prepared, annealed and filled with the explosive to be tested as hereinbefore described. The output terminals of bridge 12 are connected to the vertical sweep terminals of an oscilloscope 13, the oscilloscope being triggered by the high voltage pulser at the instant the pulser supplies energy to the bridge. The unbalance voltage of the bridge appearing on the oscilloscope is recorded by an oscillographic record camera 14 to provide a time base voltage trace of the bridge output voltage.

The high voltage pulser 11 may comprise a high voltage power supply utilized to charge a capacitor to voltages up to approximately 7,000 volts and a thyratron for discharging the capacitor across the circuit connected to the pulser 11. The circuit is initiated by closure of switch 15 which initiates the pulser 11 and places the battery E and resistance R in series across the input terminals of the bridge 12 at the same time. Switch 15 may preferably be of the mercury type which completes the circuit rapidly and without chatter. The capacitor, upon actuation of the thyratron, discharges its energy through all three branches of the circuit, but because of the lower resistance of R plus R the bulk of the capacitor discharge current flows through this path. When the capacitor has finished discharging, the high-voltage pulser represents an open circuit and only the simple Wheatstone bridge powered by storage battery E through resistance R remm'ns.

Referring now to FIG. 2, there is shown thereon a reproduction of one experimental oscillographic record obtained in the explosion of a sample of 2,2,2-trinitroethyl 4,4,4-trinitrobutyrate. As has been hereinbefore described, the oscilloscope is triggered internally by the initiation of the capacitor discharge; and after it discharges through the bridge, the output voltage of the bridge, now powered by battery B is displayed by the oscilloscope. The output voltage changes abruptly, as the explosive in the tube explodes and this changes is clearly visible on the oscilloscope trace. The temperature of the tube at any instant may be computed from the voltage recorded by the oscillogr-aphic camera and the temperature just before the abrupt change in the recorded voltage is taken as the explosion temperature. The capacitor discharge heats the tube close to its final temperature in about 20 microseconds; therefore, the explosion time or the time delay to explosion is the time at which the sharp break in the voltage trace occurs minus the 20 microsecond heating time.

In the experiment yielding the oscillographic record illustrated in FIG. 2 the following values were used:

E =24.5 volts;

R 10 ohms non-inductive resistance;

R =R =25 ohms non-inductive resistance; R =0.567 ohms;

Oscilloscope voltage range=250-310 millivolts; Oscilloscope voltage scale=10 millivolts/centimeter; Sweep speed=l00 microseconds/centimeter.

From the oscillographic record of FIG. 2 the bridge output voltage E at the time of explosion equals approximately 269 millivolts and the time delay to explosion equals 280 microseconds minus the 20 microseconds heating time or 260 microseconds. It should be noted that after the explosion the output voltage of the bridge settled at approximately 298 millivolts. This is because the tube used in this particular experiment was not completely ruptured.

The resistance R of the tube under test at the time of explosion can be computed from the bridge output voltage E and the known parameters of the circuit given above using the expression:

Substituting the values hereinbefore given into Equation 1 the resistance of the tube at explosion was 0.825 ohm.

Referring now to FIG. 3, there is illustrated a resistancetemperature curve for tubing of the type utilized in the foregoing experiment. The curve represents the ratio of the resistance of the tubing at a given temperature (R to its resistance at room temperature (R as a function of temperature. The square points illustrated on the curve represent temperature measurements made with thermal couples and resistances measured with a Wheatstone bridge while the round points illustrate temperatures measured with an optical pyrometer assuming an emissivity of 0.85, and the resistance values were obtained using a voltmeter and ammeter in a direct current heating circuit.

For the preceding example the ratio of the resistance of the tube at explosition to the resistance thereof at room temperature is:

From the curve of FIG. 3 it may be seen that the temperature of the tube and therefore the temperature of the explosive was approximately 505 C.

This information, that is the time delay to explosion at a given temperature, may be utilized impact sensitivity studies of organic high explosives as set forth in my paper The Behavior of Explosives at Very High Temperatures," published September 1961, in vol. 57 of the Transactions of the Faraday Society, pp. l6121620.

As has been hereinbefore set forth, the heavy confinement of the secondary explosive during the heating process prevents vaporization thereof and therefore prevents the formation of an insulating layer thereby allowing a secondary explosive to be directly initiated by electrical means. In the past, most initiating devices utilize an ultra-fine wire surrounded by a primary explosive. This Wire is heated to the ignition temperature of the surrounding primary explosive thereby detonating it. The primary explosive is utilized as a booster charge to initiate a secondary explosive. Such devices have the disadvantage of being mechanically fragile and costly to engineer into a suitable system and necessitate the elaborate safety devices as hereinbefore set forth.

Referring now to FIG. 4 there is illustrated an initiator comprising a thin hollow metallic tube 31, Which may be composed of stainless steel as hereinbefore described, filled with a secondary explosive in the manner hereinbe-fore described and closed at each end to heavily confine the explosive. A pair of lead wires 32 may be connected in any manner such, for exarnple, as by soldering to either end of the tube to provide for connecting the tube across a suitable initiation circuit. The initiator, when filled with 2,2,2-trinitroethyl 4,4,4-t1initrobutyrate, may be exploded by placing approximately 28 volts of direct current across lead wires 32 and may thus be utilized to initiate a larger quantity of secondary explosive. The voltage, required to initiate other types of explosives may, of course, vary depending upon the temperature required.

The initiator of FIG. 4 is mechanically rigid such that the initiating explosive cannot be dislodged by mechanical shock and is not as fragile and costly to engineer as hot Wire initiators heretofore known. Further, since the initiator requires a comparatively high power electrical source for initiation, the elaborate schemes to prevent premature explosion of the prior art devices utilizing primary explosives by radiant energy from extraneous sources need not be used.

There has been illustrated and described a new technique for initiating a secondary explosive and an apparams for obtaining the delay time to explosion of an explosive at a given temperature. It should be understood, that the test apparatus is not limited to the initiation of secondary explosives but may be utilized for the initiation of primary explosives. There has further been described a new initiator utilizing only a secondary ex- 6 plosive which is more mechanically rugged and by virtue of the utility of a secondary explosive only, is safer than the initiators heretofore known.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It should be understood, therefore, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An electrical initiation circuit for an explosive comprising, a source of high voltage pulses, a bridge circuit connected to said source, one of the legs of said bridge circuit including a hollow electrically conducting tube connected at each end to a respective terminal of said bridge circuit such that current may flow through the wall of said tube when said bridge circuit is energized, and an explosive sealed in said tube whereby when said source applies a voltage pulse to said bridge said tube is heated by current flow through the wall thereof to initiate said explosive.

2. An electrical initiation circuit for a secondary explosive comprising, a source of potential, a bridge circuit connected across said source, one of the legs of said bridge circuit including an electrically conducting hollow tube connected at each end to a respective terminal of said bridge circuit such that current may flow through the wall of said tube when said bridge circuit is energized and a secondary explosive sealed within said tube whereby when a potential is applied by said source said tube is heated by current flow through the wall thereof to detonate said explosive.

3. An electrical initiating circuit for an explosive comprising, a source of voltage pulses, a first resistance path connected across said source, a second resistance path connected across said source in parallel with said first resistance path, said second resistance path including an electrically conductive hollow tube, each end of which is connected to a respective terminal of said source such that current will flow through the wall of said tube, an explosive sealed within said tube, the resistance of said first path being substantially greater than the resistance of said second path whereby when a voltage pulse is applied by said source said tube is heated by current flow through the Wall thereof sufliciently to initiate said explosive.

4. An initiator for secondary explosives comprising a thin walled hollow electrically conductive tube, a secondary explosive sealed within said tube, electrical means connected to the ends of said tube for applying a voltage thereto whereby said secondary explosive is heated by current flowing through the wall of said tube to an extreme temperature while being held under heavy confinement to thereby initiate said explosive.

5. The initiator of claim 4 wherein said tube is stainless steel.

6. The initiator of claim 5 wherein said tube has a nominal outside diameter of 0.014 inch and a nominal inside diameter of 0.007 inch.

References Cited in the file of this patent UNITED STATES PATENTS 

4. AN INITIATOR FOR SECONDARY EXPLOSIVE COMPRISING A THIN WALLED HOLLOW ELECTRICALLY CONDUCTIVE TUBE, A SECONDARY EXPLOSIVE SEALED WITHIN SAID TUBE, ELECTRICAL MEANS CONNECTED TO THE ENDS OF SAID TUBE FOR APPLING A VOLTAGE THEREOF WHEREBY SAID SECONDARY EXPLOSIVE IS HEATED BY CURRENT FLOWING THROUGH THE WALL OF SAID TUBE TO AN EXTREME TEMPERATURE WHILE BEING HELD UNDER HEAVY CONFINEMENT TO THEREBY INITIATE SAID EXPLOSIVE. 